Brillouin Optical time Domain Analysis (BOTDA) has proven its ability to measure strain and temperature, in a distributed manner, over tens of kilometers of optical fibers. Most often the optical frequency of a constant wave (CW) probe is scanned against that of a counter-propagating pulsed pump to recover Brillouin Gain Spectrum (BGS). The strain/temperature of the interrogated fiber segment is uniquely related to the local Brillouin Frequency Shift (BFS), defined as the pump-probe frequency difference which maximizes the Brillouin gain. Normally designed to cover long to very long ranges, classical implementations of BOTDA are slow, limiting them to quasi-static scenarios.
Four main factors control the sensing speed of a BOTDA setup: (i) the round-trip time of flight through the fiber under test (FUT), which limits pump pulse repetition rate; (ii) the granularity of the frequency scanning, which affects the measurement resolution and accuracy; (iii) the switching speed of the optical frequency scanning mechanism; (While on the order of milliseconds in classical BOTDA setups, this factor was practically eliminated by the Fast BOTDA (F-BOTDA) method, disclosed in US Patent Publication No. US20140022536A1 which is incorporated herein in its entirety. The F-BOTDA method enables an almost instantaneous (nanoseconds) frequency transitions.) and (iv) the number of averaging.
The first two factors impose an ultimate bound on the speed by which a BOTDA acquires a single BGS. The last factor deals with averaging, which is normally needed for two reasons: (a) to improve the signal to noise ratio (SNR), which, however, may not be required for relatively short fibers (<1 km), where strong pump pulses provide high gains without giving rise to detrimental nonlinear optical effects; and (b) to overcome the inevitable ‘polarization fading’, a term referring to the fact that the Brillouin interaction depends on the degree of parallelism of the States of Polarization (SOPs) of the pump and probe. However, in a standard, weakly birefringent FUT, these two SOPs hover around each other, resulting in highly non-uniform gain along the FUT, and consequently, fiber segments with minimum gain and poor SNR. Most often, this problem is eliminated by scrambling the state of polarization (SOP) of one of the interacting waves and averaging the sensor readings over multiple pump pulses, until a sufficiently high SNR is recorded along the full length of the FUT. This process significantly slows down the BGS acquisition speed. An alternative solution to the polarization fading problem, which reduces the acquisition speed by only a factor of two, involves the use of a fast polarization switch, where two orthogonal SOPs are sequentially launched into the fiber, and the resulting readings added. Recently, several new techniques have been introduced to eliminate the polarization fading using polarization diversity.
A paper titled “Simple method for the elimination of polarization noise in BOTDA using balanced detection of orthogonally polarized Stokes and anti-Stokes probe sidebands” published in 23rd International Conference on Optical Fiber Sensors (2014) discloses frequency scanning BOTDA implementation in which two probes interacted with a single pump so that one experienced gain while the other loss. The two probes, of different frequencies, were made orthogonal by separating them using a Dense Wavelength Division Multiplexing (DWDM) coupler, having a mirror on one output port and a Faraday mirror on its other port.
Another paper titled “Polarization diversity for Brillouin distributed fiber sensors based on a double orthogonal pump” in 23rd International Conference on Optical Fiber Sensors (2014) discloses another technique which employs the phase of the complex BGS rather than its magnitude. This method does not use frequency scanning but rather a single frequency interrogation, by two pairs of pumps and probes. Orthogonality of the different frequencies pumps was obtained by passing them through a Differential Group Delay (DGD) module, which differentially transforms their (originally) same polarization into two orthogonal ones.